1. Field of the Invention
The present invention relates to an exchange which uses the synchronous transfer mode (STM) and which performs processing for exchange of voice and other various types of data. Usually, in time division multiplexing, it is possible to assign a number of time slots according to the bandwidth of transmission of the data. Accordingly, image data, which requires a broader transmission bandwidth than voice data, can also be transmitted by time division multiplexing together with voice data etc. There is a demand, however, to enable more efficient processing for exchange of data having various transmission bandwidths.
2. Description of the Related Art
As will be explained in detail later by referring to the drawings, an exchange of an example of the related art is constituted by a time switch (TSW), highway interface units (HWI) for multiplexing and demultiplexing the data, terminal common units, terminal cards, highways between the time switch and the highway interface units, and highways between the highway interface units and the terminal common units.
In such an exchange of the related art, when an increase of the terminal cards is necessary, once for example the sum of the number of time slots for the bandwidths of the terminal cards to be accommodated by one terminal common unit reaches the number of time slots on the highway between the highway interface unit and the terminal common unit, the terminal common unit can no longer connect more terminal cards. In this case, the terminal cards to be increased are accommodated by the terminal common unit by increasing the number of highways and terminal common units.
Generally, further, there is an upper limit on the number of the terminal cards which can be accommodated by any single terminal common unit. For this reason, there are cases where the-sum of the number of time slots for the bandwidths of the terminal cards which can be accommodated by a terminal common unit is smaller than the number of time slots on the highway between the highway interface unit and the terminal common unit. Namely, even if it is possible to increase the number of terminal cards from the standpoint of the capacity of the highway, the number of terminal cards cannot actually be increased due to the limit on the number of terminal cards which can be connected to the terminal common unit. Therefore, it is necessary to increase the number of terminal common units and highways to accommodate, by the terminal common unit, the terminal cards to be increased.
Namely, in an exchange of the example of the related art, even if there are idle time slots on a highway, the number of terminal cards cannot necessarily be increased, so there has been the problem in that the efficiency of use was low.
In consideration with the above problem, an object of the present invention is to achieve effective utilization of the highway.
To attain the above object, the present invention provides an STM type exchange, including a time switch, for performing exchange processing of time division multiplexed data, wherein a ring highway is connected via a terminal common unit to an upstream highway and a downstream highway coupled to this time switch via a highway interface unit or directly and wherein a plurality of terminal cards are connected to this ring highway. Each terminal card is provided with an add/drop unit which drops and adds the data from and to an assigned time slot on the ring highway according to control information indicating time slot assignment information determined by the control unit and adds the data and with a card control unit which controls the add/drop unit. By this, it is possible to improve the efficiency of use and to further facilitate an increase of the number of terminal cards in an exchange using STM.